This invention relates to new roadway marking systems, and, more particularly, to an enhancement to roadway marker paints that renders the road markers more distinguishable from the adjacent and/or underlying pavement when viewed at microwave/millimeter wave radiometric frequencies. The invention also relates to a method for ascertaining the presence of roadway markers and information coded in such markers by application of passive radiometric energy sensitive inspection apparatus within an electronic control and/or warning system.
Roadway marking systems have long been used to provide vehicular equipment operators with pertinent information through the medium of roadway markers. As example, the white stripe painted on the roadway in front of a stop sign, familiar to the lay reader, provides a vehicle operator, the driver, with a physical limit or boundary that the driver""s approaching vehicle should not exceed in coming to a full and complete stop in obedience to the stop sign. On multi-lane roadways, the lanes are delineated by roadway markers. And, at major airports service roads and corridors are often distinguished, in addition to signal lamps, by painted lines marking the borders to the service road, providing a visible guide for the pilot. The foregoing are but a few of the most common applications.
In more recent experience, roadway markers have also been adapted as part of vehicular guidance and control systems. The information provided by the roadway markers is used to automatically issue an alarm or steer and/or position a moving vehicle. Sensors on the vehicle detect a marked path along a roadway and the associated control equipment on the vehicle is able to automatically correct the vehicle""s steering should the sensor detect the vehicle""s departure from the marked path. From time to time newspapers report of experimental automobile control systems that are intended to automatically control and guide a vehicle""s travel along a highway, eliminating the need for the driver""s complete attention.
Examples of the foregoing appear in the patent literature. The system in U.S. Pat. No. 5,202,742 to Frank et al carries a laser radar carried on the vehicle to detect reflective markers along a roadway. The laser beam is scanned over the roadway and the associated detectors, which receive light reflected from the roadway markers, are used by associated control equipment on board the vehicle to guide the vehicle relative to the roadway markers. Another system is presented in U.S. Pat. No. 4,947,094 to Dyer et al. In the Dyer system a linear charge coupled device (CCD) carried by an industrial warehouse vehicle, such as a forklift, monitors the position of a track, such as formed by a painted line on the warehouse ceiling overlying the roadway. The CCD images that line and that imaging allows the control equipment in the system to steer the industrial vehicle along the track.
Though painted white stripes are often used, the better roadway markers are formed of a thermoplastic material, supplied by the manufacturer as minute plastic granules or beads, that is heated to place the material into the liquid form, which can flow. Often small spheroidal glass particles are mixed into the ingredients as part of the liquid. That hot liquid is coated or extruded in a thin strip onto the roadway surface, where the plastic material is allowed to cure, that is, solidify and harden. The plastic material is designed to seep into the rough surface and pores characteristic of pavement materials, such as cement and asphalt, and hardens to form a firm grip or bond to the pavement.
Such marker is relatively wear resistant, enduring the heavy pounding and friction of automobile tires. It resists the effects of snow and rain. It also resists to the deleterious effects of sunlight, including that from ultra-violet radiation. And it maintains its color for years, ensuring a visible contrast with the surface of the roadway. Although those who apply the marker to the roadway refer to the thermoplastic film simply as xe2x80x9cpaintxe2x80x9d, an analogy to house paint, a reference that carries forward in the subsequent description, roadway markers are understood as a serious field of endeavor.
It is noted that the exact composition of the various thermoplastic ingredient materials suitable for pavement marker application, not known to the present applicants, is well known to those skilled in the road marker art. As becomes apparent those details are not necessary to an understanding of the present invention and, hence, need not be further described. Those interested in learning more on that subject, may make reference to the technical literature in that field.
The foregoing marker and control systems make use of reflected light, that is, the visible region of the electromagnetic energy spectrum, and that light originates either naturally in the environment or is generated by a light source in the detection system. Other forms of energy, though not perceptible directly by human senses, are known and have also been applied in detection schemes. As one finds from the scientific literature, the electromagnetic energy spectrum extends over a wide range of wavelengths, extending at least from the shortest wavelengths, those in the ultra-violet region and below, to and beyond the longest wavelengths in the infra-red regions. One finds visible light in this spectrum, a region which human eyes are able to detect and which enables our vision, and also radio waves. The microwave spectrum lies in a portion of that radio spectrum; and in an end portion of that microwave spectrum, one finds the millimeter wave region.
Microwave and millimeter wave energy is emitted naturally from all objects. It is also incident on our Earth from outer space, and from the Earth""s atmosphere, a gaseous object, irradiating, among other things, the roadways on which we travel. Since outer space is very cold, approximately four degrees Kelvin, and since the amount of energy emitted is proportional to the emitting objects physical temperature, very little energy is incident from outer space. For the most part the incident microwave/millimeter wave energy incident on the roadway is from the atmosphere itself, which serves or acts roughly speaking like a forty degree Kelvin emitter at a 94 GHz frequency. This energy, in part, is reflected from the materials on which it is incident, including the roadway and markers on the roadway. Those materials also emit a like kind of energy, and, since those materials are typically at xe2x80x9croom temperaturexe2x80x9d, 300 degrees Kelvin, they appear warmer, higher in temperature, than those materials that principally reflect the xe2x80x9ccold skyxe2x80x9d.
In fact, according to Planck""s radiation law, any perfectly absorbing body emits radiation at all frequencies of the energy spectrum. For most natural objects in our environment, such radiation is relatively high in the infra-red region of the energy spectrum, proportional to the fourth power of the object""s physical temperature. At microwave/millimeter wave frequencies, the energy is much less, varying only directly with the temperature. Though less intense, that microwave/millimeter wave energy is detectable and measurable with properly designed microwave/millimeter wave radiometers.
Microwave/millimeter wave radiometric detectors are microwave/millimeter wave receivers that detect the total power received. A microwave/millimeter wave radiometer is, in effect, a highly sensitive total power receiver. The receiver receives its signals from a directional antenna, such as a microwave/millimeter wave horn antenna, whose receiving xe2x80x9cfootprintxe2x80x9d or field of view is directed at the element or area to be observed. The magnitude of the signal received by the radiometer is proportional to the temperature of the object under observation, and/or the temperature reflected by the object, depending upon the percentage and types of objects within the antenna""s footprint and the object""s emissivity xcex5, the latter being equal to (1xe2x88x92xcfx81), where xcfx81 is the object""s reflectivity. Radio astronomers have long used radiometric detectors to scan the heavens to detect planetary bodies and stars.
For convenience, the term, microwave/millimeter wave, is hereafter sometimes abbreviated to MMW. Thus, when used to modify the term radiometer, the abbreviated term distinguishes the radiometer discussed in connection with the present invention from other known types of radiometers, such as infra-red radiometers.
Radio astronomers also earlier determined the existence of xe2x80x9cpropagation windowsxe2x80x9d through the atmosphere for millimeter wave energy, frequencies in the 30 to 300 GHz range, at which the attenuation is relatively modest in both clear air and fog. That is, transmission of millimeter wave energy from outer space at those xe2x80x9cwindowxe2x80x9d frequencies are not attenuated in propagating through overlying clouds to their ground based radio telescopes as greatly as adjacent higher or lower frequencies in the millimeter wave region about that frequency. Similarly, the atmosphere does not emit as much energy in these windows, and, therefor does not xe2x80x9cwash outxe2x80x9d or overpower signals from space. These windows occur at 35 GHz, 94 GHz, 140 GHz and 220 GHz.
Imaging of ground objects, including aircraft runways, using MMW radiometric energy and MMW radiometric detectors, was proposed in the past. U.S. Pat. No. 3,725,930 to Caruso uses a microwave radiometer to detect a pattern of radiometric energy reflecting markers on an airport runway. Metal, such as iron, is known as a good MMW radiometric reflector; it is of high xcfx81. In Caruso""s system, wedge shaped metal plates, of a size between two feet and twenty feet in diameter, are placed on the runway as markers to provide a surface tilted up from the runway surface as presents a ramp to the oncoming airplane. Microwave energy from a portion of the cold sky is thereby reflected toward radiometric detectors carried on the taxiing aircraft, and, thus, such markers stand out from the surrounding landscape as cold looking.
Although Caruso does not expressly describe the height of the metal ramps thus employed, one appreciates that any protrusion from the roadway surface poses an impediment to the taxiing airliner and other airport vehicles using the runway. At a minimum such obstacles would cause the aircraft to sustain a series of bumps, as might disturb the passengers, and, if great enough in height, would present an obstacle that would render Caruso""s system impractical of application.
Placed on the runway, the large metal plate is visible to view and does not have the anonymity of a painted stripe. It can easily be removed from the roadway surface. The metal plate thus provides an attraction to those pranksters or unscrupulous persons who would detach and sell the metal as scrap, disabling a key element of the marker system. That visibility is an unfortunate practical drawback.
Further, in an article published by the assignee of the present invention xe2x80x9cPassive Millimeter-Wave Imagingxe2x80x9d, Yujiri et al, Quest Magazine, TRW, Inc. 1990/1991, Vol. 13, No. 2, the authors, including one of the present inventors, present two dimensional pictures of the radiometric image of an airport, harbor and other scenes taken with experimental apparatus containing radiometric detectors tuned to receive energy at a frequency of 94 GHz. Though the images are of low resolution, the viability of radiometric inspection is demonstrated as technically feasible and urged as a soon-to-be practical means to achieve images of scenes, despite rain and fog weather conditions The article predicts additional technical development, and, though offering suggestions for radiometric imaging, the article offers no guidance on improving roadway marker systems.
Accordingly, an object of the present invention is to provide a new method for detecting roadway markers, one that uses MMW radiometers;
It is another object of the invention to provide a roadway marker detection and control system that is passive and does not require transmitting apparatus that emits electromagnetic energy;
It is a further object of the invention to provide a roadway marker paint having enhanced contrast with the adjoining roadway surface at MMW radiometric frequencies, a true xe2x80x9cmicrowave/millimeter wave radiometricxe2x80x9d paint; and
It is a still further object of the invention to provide an enhanced radiometric paint that may be applied to roadway surfaces using existing unmodified paint striping equipment.
In accordance with the forgoing objects, an improved roadway marker detection method employs a vehicle or other wheeled platform capable of movement along the roadway surface. The vehicle supports a downwardly inclined looking radiometer a short distance above the roadway. The vehicle is then moved along the roadway and the MMW radiometric energy, reflected and/or emitted from the various patches of road surface and viewed as the vehicle progresses forward, is monitored and may be displayed. Any roadway markers encountered along the way are detected by a significant decrease in energy received by the radiometer as a result of viewing energy from the xe2x80x9ccoldxe2x80x9d sky.
Unlike prior systems, the foregoing method distinguishes markers in rain or fog and in day or at night, with wet or snow covered roadways. Markers are detected passively. It is not necessary to emit electromagnetic radiation into the environment to make the detection, minimizing risks to personnel and avoiding possible interference to other electronics equipment. Moreover, since artificial illumination is unnecessary, the roadway infrastructure requirements are simplified.
In accordance with another aspect to the invention, a new roadway marker paint possesses enhanced MMW radiometric energy reflection characteristics obtained by incorporation of metal particles and/or high dielectric particles within the thermoplastic material of ordinary road marker paint. A preferred embodiment of the enhanced radiometric paint includes size 20 or 30 iron shot with the iron shot comprising approximately 30% of the paint mixture by volume.
When the foregoing inspection procedure is made of markers constructed in that way, the decrease in detected temperature is larger than before on some surfaces, such as asphalt, although the change is not as great on concrete. This evidences an increased radiometric contrast between the marker and the adjacent pavement. As a benefit the new radiometric paint may be applied to the roadway using existing paint striping equipment. With the increase in contrast between the radiometric energy reflecting characteristic of the marker and that of the roadway, a more pronounced measurement is available for detection. With increased signal, the signal to noise ratio is improved and detection is possible even if the sensitivity of the detector lowers with age.
The foregoing and additional objects and advantages of the invention together with the structure characteristic thereof, which was only briefly summarized in the foregoing passages, becomes more apparent to those skilled in the art upon reading the detailed description of a preferred embodiment, which follows in this specification, taken together with the illustration thereof presented in the accompanying drawings.